Filtration devices comprising a hollow plunger which slides within a hollow tubular vial are frequently employed in laboratory and other environments to remove solids from a liquid sample. For example, biological samples such a blood, mucus or urine may be filtered (to remove contaminants or unwanted protein for example), the filtrate being subsequently used to perform medical or other tests.
In such devices, the hollow plunger typically has an aperture at one end, which is fitted with a filter membrane. The liquid sample to be filtered is initially held in the tubular vial, which is open at one end and closed at the other. The hollow plunger is inserted into the tubular vial, with a sealing contact being formed between the exterior of the hollow plunger and the interior of the tubular vial. The hollow plunger is then depressed into the tubular vial, with the sealing contact forcing the liquid sample through the filter membrane and into the interior of the hollow plunger. The filtrate is then held inside the plunger, until it is required for subsequent processing. The plunger may be fitted with a cap to prevent the filtrate escaping. When the filtrate is required, the cap may be pierced using a syringe or other device in order to extract the filtrate. U.S. Pat. No. 4,800,020 describes an example of a filtration device of this type.
The sealing contact mentioned above is typically formed by using a rigid O-ring or similar seal which is located onto the plunger body during assembly. This makes assembly of the filtration device complicated, particularly in view of the fact that such seals are typically small and difficult to handle during automated or manual assembly; further, locating grooves or other structures may be required in the plunger body, complicating the manufacture of the plunger body.
Further, in order for the sealing contact mentioned above to be effective, a relatively tight fit between the O-ring and the interior walls of the tubular vial is required; if the fit is too loose, the sealing contact becomes ineffective, allowing the liquid sample to escape around the sides of the plunger, rather than though the filter membrane. On the other hand, if the fit is too tight, the force required to push the plunger into the tubular vial may become excessive, making the device difficult to operate; furthermore, the tubular vial may break under excessive force. Accordingly, a high degree of accuracy in the external diameter formed by the O-ring and the internal diameter of the tubular vial has been required.
As a result, the tubular vial has typically been manufactured from a plastics material, since such material is relatively easy to manufacture to a high degree of accuracy (for example, by injection moulding), and is relatively resistant to breakage. However, plastics materials are often susceptible to leaching by the liquid sample, which may result in the liquid sample being contaminated with impurities from the plastics material prior to filtration, which impurities may not all be removed during the filtration process. It would therefore be desirable to use a tubular vial made of a more inert material, less susceptible to leaching by the liquid sample, such as glass. However, glass vials typically have a greater variation in internal diameter from vial to vial than plastics vials, making them unsuitable for use with the plungers described above. In addition, conventional glass vials are generally not as strong as plastics vials, particularly when subjected to expansive forces exerted by compressed plunger seals or fluids being compressed as the plunger is forced into the vial. Embodiments of the present invention at least mitigate some of the problems of the prior art.